1. Field of the Invention
The present invention relates to a communication system using a mobile Internet protocol (mobile IP) scheme (mobile IP communication system), and more particularly to a method for providing a multicast service according to handoff of a mobile node (MN) functioning as a source node.
2. Description of the Related Art
In general, when an MN hands off in using a mobile IP scheme, time delay caused by the handoff results in packet data loss. In an attempt to minimize such packet data loss, a fast handoff mode has been proposed, wherein time delay needed by an MN to perform handoff is minimized so that packet data loss is minimized accordingly.
In a multicast service scheme provided by a mobile IP communication system, packet data is multicast using an IP address reserved for a multicast service of packet data, i.e. a multicast service IP address. Particularly, when packet data is transmitted via a network in the multicast service scheme, a receiver receives the packet data only after referring to a destination IP address of the packet data and confirming that it is identical to a multicast service IP address of desired packet data. In the multicast service scheme, a transmitter (i.e. source node) can simultaneously transmit packet data to a number of receivers in a single process (i.e. it can multicast packet data). This avoids waste of network resources caused by repeated transmission of the same data packet.
When the multicast service scheme is used, paths must be established between a transmitter, which transmits packet data, and receivers, which receive the packet data, in a tree scheme. The tree scheme is generally classified into a source-based tree scheme and a shared tree scheme. When the source-based tree scheme is used to establish paths between the transmitter and receivers, optimal paths are maintained between them and time delay decreases. In contrast, when the shared tree scheme is used to establish paths between the transmitter and receivers, a single path is shared in a predetermined range from the transmitter to the receivers.
The above-mentioned fast handoff scheme is applied when MNs transmit/receive packet data using a conventional mobile IP scheme. When handoff is performed while an MN provides or is provided with a multicast service using the multicast service scheme, packet data loss due to time delay caused by the handoff occurs in the same manner as when MNs transmit/receive packet data using a conventional mobile IP scheme.
FIG. 1 briefly shows handoff operation of an MN, which provides a multicast service in a conventional IP communication system.
Prior to a description of FIG. 1, it is to be noted that various studies are currently in progress to minimize time needed by an MN to hand off, which provides a service in a conventional mobile IP scheme or fast mobile IP scheme, and resulting packet data loss. However, few studies have been conducted to minimize time needed by a multicast service-providing MN (source node) to hand off, and this results packet data loss.
Referring to FIG. 1, while performing communication with an access router (AR) 110 (AR#1) in its service region, an MN 100 for providing a multicast service (i.e. source node) hands off into a service region of another AR 120 (AR#2). As the MN 100 hands off from the AR 110 to the AR 120, receivers receiving the multicast service from the MN 100, particularly, receivers 150, 160, and 170 (R#1, R#2, and R#3) must newly construct a multicast tree on the MN 100 (i.e. source-based tree), because the source node providing the multicast service has handed off.
However, it takes a long time to reconstruct a source-based tree in current mobile IP communication systems, because little consideration is given regarding how to reconstruct a source-based tree when a multicast service-providing source node hands off. Particularly, when a source node performs handoff, a new AR, to which the source node has handed off, must recognize that the source node is a multicast service provider, but receivers cannot recognize the handoff of the source node. As a result, multicast service data, received during time delay caused by handoff and as the receivers join a new source-based tree following the handoff, is inevitably lost. Even when a fast handoff scheme is used to minimize time delay caused by handoff of the source node, the source node is still obliged to reconstruct a source-based tree, in order to provide a multicast service after performing the handoff. The resulting loss of multicast service data is inevitable.
In summary, time delay caused by handoff of a source node and reconstruction of a source-based tree result in loss of multicast service data and degrades the overall performance of mobile IP communication systems.